The typical electrical power supply system for automotive vehicles is a so-called constant voltage system having a normal rated voltage of twelve volts. It comprises a single storage battery, a generator and a regulator to maintain the generator output voltage and current within certain limits. The generator, which typically takes the form of an alternator with a rectifier, is driven at variable speed by the vehicle engine and supplies the DC power to the load circuits and charges the battery. The regulator is typically a switching system for the field coil of the alternator across the battery to regulate the generator output voltage to the desired value and to limit the generator current to a desired value.
Dual battery systems are also known for use in automotive vehicles. Such systems are useful in vehicles which have different load devices adapted to be energized at two different voltages. For example, the system may comprise two batteries with a first load circuit including load devices designed to be powered at the rated voltage of one battery and a second load circuit including load devices designed to be powered at twice the rated voltage of one battery. Various arrangements have been proposed for charging the two batteries from the generator and for providing the dual voltage output to the load circuits. It is advantageous to connect the two batteries in series with the high voltage load circuit across the series batteries and the low voltage load circuit connected across the lower battery of the series combination. This requires appropriate charging circuits for the respective batteries; it also requires special overvoltage protection to prevent excessive voltage across the lower battery under certain circumstances such as a defective upper battery.
In the prior art, a dual voltage system with two batteries in series is disclosed in the Seike U.S. Pat. No. 3,710,226 granted Jan. 9, 1973. One of the batteries is charged by two cooperating phases of the alternator, the output of the two phases being controlled by a field regulator for the alternator. The other battery is charged by the third phase of the alternator, the third phase output being regulated by a series regulator.
In the Follmar U.S. Pat. No. 4,044,293 granted Aug. 23, 1977, a dual voltage system with two batteries is described in which the alternator charges the series combination of the batteries. A separate regulator is provided for one of the batteries and when it is fully charged the regulator is effective to bypass that battery out of the circuit so that the alternator is connected only with the first battery.
Another dual voltage, dual battery system is disclosed in the Himmler U.S. Pat. No. 4,047,088 granted Sept. 6, 1977. In the Himmler system, the output of a three phase alternator-rectifier is used to charge two twelve volt batteries which are connected in series across a twenty-four volt load circuit. One of the batteries, which is connected with a twelve volt load circuit, is charged by an auxiliary charging circuit connected across one of the phase windings of the alternator.
The Taylor U.S. Pat. No. 4,210,856 granted July 1, 1980, discloses a dual voltage system provided with two different batteries. One battery is charged from an alternator-rectifier and the two batteries in series are charged from a DC-to-DC converter energized from the first battery.
A general object of this invention is to provide a dual battery, dual voltage electrical system for automotive vehicles which overcomes certain disadvantages of the prior art.